An electron accelerator is a powerful tool for fundamental and applied research. One important application of an electron accelerator is on the generation of electromagnetic radiation. A typical electron radiation apparatus consists of three major components, an electron emitter, an electron accelerator, and a radiation device. Low-energy electrons are generated by the electron emitter and further accelerated to acquire high energy by the electron accelerator following the electron emitter. The high-energy electrons are then injected into the radiation device to generate an electron radiation. In practice, different radiation devices are applied to different radiation apparatus. For example, the radiation device of a free-electron laser (FEL) is an undulator; the radiation device of a Smith-Purcell radiator is a grating; the radiation device of a Cherenkov radiator is a dielectric, and the radiation device of a backward wave oscillator is a slow-wave waveguide.
The performance of electron radiation strongly depends on the characteristics of the driving electrons. It is known that the mechanism of electron radiation has two distinct regimes in terms of the electron bunch length relative to the radiation wavelength. Briefly if the electron bunch length is much longer than the radiation wavelength, the electrons generate incoherent radiation from the radiation device with a spectral energy linearly proportional to the electron current or to the total number of electrons. This incoherent radiation often occurs in synchrotron radiation. If the electron bunch length is much shorter than the radiation wavelength, the electrons radiate coherently in the so-called superradiance regime, where the spectral energy of the radiation has a quadratic dependence on the electron current or on the number of electrons.
Specifically, when a single electron transmits through a radiation device, the radiation energy carries the spectral characteristics of the radiation device. Regardless of the nature of the radiation device or scheme, let (dW/dω)1 denote the spectral energy emitted from a single electron, where W is the radiation energy, ω is the angular frequency of the radiation, and subscript 1 denotes “a single electron”. The total energy radiated from a stream of electrons in a radiation device is highly dependent on the electron bunch time τb relative to the radiation period 2π/ωr or on the electron bunch length σb relative to the radiation wavelength λr for relativistic electrons. If N electrons are uniformly distributed over several radiation wavelengths, the electrons radiate with all possible phases between 0 and 2π when transmitting through a radiation device, yielding a spectral energy expressed by:(dW/dω)inc=N(dW/dω)1  Equation (1)Because not all the radiation fields from the electrons are added up constructively, this radiation process is incoherent. However, if Nb electrons are distributed in a delta function in time or τb˜0, all the radiation fields from the electrons are in phase and summed up constructively, resulting in a total spectral energy equal to Nb2(dW/dω)1. This radiation process is dubbed as superradiant emission or superradiance, having a radiation spectral energy proportional to the square of the number of electrons. To account for a finite electron bunch length τb, the total spectral energy from an electron bunch is expressed by:(dW/dω)SR=Nb2(dW/dω)1Mb2(ω),  Equation (2)where Mb(ω) is the Fourier transform of the electron pulse-shape function with a unitary peak amplitude. If Npb such electron bunches repeat periodically at a rate ωpb/2π, the total radiated spectral energy is given by:
                                                        (                                                ⅆ                  W                                /                                  ⅆ                  ω                                            )                                      SR              ,              pb                                =                                    N              b              2                        ⁢                                                            N                  pb                  2                                ⁡                                  (                                                            ⅆ                      W                                        /                                          ⅆ                      ω                                                        )                                            1                        ⁢                                          M                b                2                            ⁡                              (                ω                )                                      ⁢                                          M                pb                2                            ⁡                              (                ω                )                                                    ,                                      Equation          ⁢                                          ⁢                      (            3            )                          ⁢                                                  where                                                                            M            pb            2                    ⁡                      (            ω            )                          =                                            sin              2                        ⁡                          (                                                N                  pb                                ⁢                                  πω                  /                                      ω                    pb                                                              )                                                          N              pb              2                        ⁢                                          sin                2                            ⁡                              (                                  πω                  /                                      ω                    pb                                                  )                                                                                  Equation          ⁢                                          ⁢                      (            4            )                          ⁢                                      is the coherent sum of the radiation fields from all the micro-bunches and has an unitary peak amplitude at the frequencies ω=mωpb (m=1, 2, 3 . . . ). To have a large radiation spectral energy, one would like to have a short bunch length (Mb2(ω)˜1) and match the radiation frequency to one of the harmonics of the bunch frequency (ω=mωpb). For ω=mωpb and Mb2(ω)=1, the radiation spectral energy becomes (dW/dω)SR,pb=Nb2Npb2(dW/dω)1, which indicates a quadratic dependence on the electron current. In many applications, a narrow spectral linewidth is important. For a short electron bunch, Mb2(ω) is usually a broad-band function. The spectral linewidth of Mpb2(ω) at ω=mωpb is given by ˜ωpb/Npb, which, for a large number of periodic electron bunches Npb, could be much narrower than the intrinsic spectral linewidth of a radiation device governed by (dW/dω)1. In this limit, the spectral linewidth of (dW/dω)SR,pb∝(dW/dω)1Mpb2(ω) is approximately that of Mpb2(ω) or ˜ωpb/Npb for a radiation frequency equal to ω=mωpb.
Refer to FIG. 1, which shows a schematic diagram of an electron radiation apparatus in the prior art. The electron radiation apparatus 100 includes a pulsed driver laser system 10, an electron emitter 13, an electron accelerator 14, and a radiation device 17. The assembly of the electron emitter 13 and the electron accelerator 14 is usually called a photocathode electron accelerator 12. The electron emitter 13 emits an electron pulse 15 when incident by a laser pulse 11 from the pulsed driver laser system 10, and the photoemitted electron pulse 15 is immediately accelerated to become a high-energy electron pulse 151. The electron radiation apparatus 100 further includes a beam transport system 16, wherein the high-energy electron pulse 151 is delivered through the beam transport system 16 and injected into the radiation device 17 for generating a radiation pulse 18. The beam transport system 16 could contain focusing and bending elements for electron-beam delivery or accelerators for further electron acceleration. Since the length of the laser pulse 11 and thus the length of the electron pulse 15 and the high-energy electron pulse length 151 are usually much longer than the wavelength of the radiation pulse 18, the spectral energy of the radiation pulse 18 can only follow the incoherent radiation Equation (1) in a synchrotron radiation device or could follow the Equations (2-4) in a radiation device equipped with some electron bunching scheme.
It is known that the electron superradiance can sometimes be generated from a single-pass type of radiation device such as Smith-Purcell radiator, a Cherenkov radiator, or an undulator radiator. The mechanism thereof is that the initially incoherent radiation field acts back the electrons to gradually form electron micro-bunches in the radiation device. The radiation power saturates at a high level when those micro-bunches are formed in the radiation field. However most single-pass electron radiators do not have enough radiation gain to reach the superradiance regime and saturate the radiation power. Electrons in a FEL oscillator can also form periodic bunches and radiate efficiently when the electromagnetic signal gradually builds up between the two resonator mirrors of the laser oscillator. Unfortunately a FEL oscillator is more complex than a single-pass undulator radiation device.
To assist the electron self-bunching, a short undulator with a drift distance or with a magnetic chicane is sometimes installed in front of a FEL oscillator. This short undulator is usually called an optical klystron, because a klystron as a microwave amplifier has a structure for inducing electron bunching. However, the optical klystron can only introduce very limited density modulation to an electron beam due to its weak spontaneous-radiation field in the short structure. In order to overcome the drawback of the optical klystron, a very long undulator is provided for a high-gain single-pass FEL for generating self-amplified spontaneous emission (SASE). Although this single-pass scheme avoids using resonator mirrors for a FEL, the undulator length has to be significantly longer than a conventional one and the electron beam quality (emittance, energy spread, and current density) driving the SASE FEL has to be much superior to those for an ordinary FEL oscillator. Furthermore, this single-pass FEL amplifies shot noises in the electrons and generates noisy spectral and temporal outputs.
To solve the noisy-output problem of a SASE FEL, a laser seeded modulator undulator in front of the SASE FEL is used to induce periodically bunched electrons with a bunch frequency equal to the sub-harmonic of the radiation frequency of the SASE FEL. However, this so-called high-gain harmonic-generation (HGHG) technique requires an investment on the modulator undulator and on a frequency-specific seed laser source. To reach a radiation wavelength much shorter than the laser wavelength, the HGHG scheme needs a large number of cascaded modulator and radiation undulator structures.
All the coherent electron radiation devices in prior arts simply adopt the electron current as is from an existing accelerator and rely on complex, expensive, or inefficient schemes external to an accelerator to bunch electrons for generating efficient electron radiation. Therefore, it is an intention of the present invention to provide a new electron accelerator and a new coherent radiation apparatus using such an accelerator to overcome the above-mentioned drawbacks in prior arts.